What is his rationale on double pane over triple pane?
If you are okay with the slightly darker window, triple pane is a great investment.
Triple pane will have a bigger impact on overall wall performance than most give it credit for.
A standard 2x4 (R-13) wall will have an average R-Value of about 6-8 when you figure in the thermal bridging of the studs and the and the less than optimal performance.
If you were to re-calculate the R-Value of that entire wall assembly (window included and figuring that the wall to window split is 75% - 25%), the double pane window (U-Factor of 0.30) will net and average entire wall R-Value of 5.5.
That same wall with triple pane (U-Factor 0.21) windows installed will bump it up to 6.3. That is a 15% increase in the thermal resistance of the entire wall.
In theory, you would think that a 40% increase in the window performance would only net about a 10% increase given the 25% of the wall it represents but it is not a linear relationship.
Are you bored yet.....